Flash Defrost System

ABSTRACT

A vapour compression refrigeration system includes a compressor ( 1 ) arranged to re-circulate refrigerant through a condenser ( 2 ), an expansion device ( 4 ) and an evaporator ( 5 ). To achieve rapid thermodynamically efficient defrosting of the evaporator,  hot refrigerant from the condenser is stored in a defrost receiver ( 6 ) before passing through the expansion device ( 4 ). In a defrost phase, a valve arrangement  ( 7 - 10 ) forms a closed defrost circuit connecting the evaporator ( 5 ) to the defrost receiver ( 6 ) via defrost valve ( 10 ) to allow hot fluid to pass from the defrost receiver to the evaporator and liquid refrigerant  in the evaporator flows to the defrost receiver ( 6 ) via drain valve ( 9 ). In a pre-defrost phase, the valve arrangement closes the fluid input to the evaporator ( 5 ) and the compressor operates to partially evacuate the evaporator before the evaporator is connected to the defrost receiver, so that flash flooding of the evaporator with hot vapour occurs. A phase change medium ( 11 ) may be included to store heat from the condenser output and return it to the evaporator during defrost. Additional heat may be supplied to the defrost liquid to further increase the defrost speed.

TECHNICAL FIELD OF THE INVENTION

This invention relates to a flash defrost system for defrostingevaporators in vapour compression refrigeration systems. As will beexplained more fully herein, the invention is applicable to directexpansion, flooded evaporator and liquid overfeed refrigeration systems.

BACKGROUND

In many applications of vapour compression refrigeration systems anevaporator is used to cool air, inter alia, in chiller rooms,supermarket chilled display cabinets, domestic freezers and air sourceheat pumps. In such applications the external surfaces of the evaporatorbecome covered in ice during operation due to condensation and freezingof water vapour in the atmosphere. Ice formation adversely affects theheat transfer performance, and the power consumption of the compressorrises to compensate for loss of evaporator efficiency. All such systemsare therefore designed to periodically defrost the evaporator in orderto restore performance and minimise running costs. Common methods ofdefrost include, in order of defrost speed: discontinuation of therefrigeration process whilst electrical heaters attached to theevaporator are used to melt and release the accumulated ice;discontinuation of the refrigeration effect but, with the compressorstill running, diversion of the hot gas output along an extra line tothe evaporator for a time sufficient to melt and release the ice;discontinuation of the refrigeration effect and the use of ambient airto melt the ice. To minimise temperature rises in the refrigeratedproducts the time of defrost needs to be short, so that electricaldefrost is most commonly used in food applications. However, electricaldefrost and hot gas defrost also incur a cost penalty in terms of extraenergy used.

WO 2009 034 300 A1 discloses an ice maker which includes a vapourcompression refrigeration system having multiple evaporators. Relativelyhot refrigerant from a condenser flows through a defrost receiver beforepassing through the evaporators. Individual evaporators can be defrostedby means of a valve system which connects the evaporator to the defrostreceiver to allow hot fluid to pass thermosyphonically from the defrostreceiver to the evaporator and liquid refrigerant in the evaporator toreturn by gravity to the defrost receiver. However, in such a system thelength of the defrost period is relatively unimportant since theremaining evaporators will continue to operate.

The present invention seeks to provide a new and inventive form ofdefrost system which is capable of providing more rapid andenergy-efficient defrosting of the evaporator than has hitherto beenpossible.

SUMMARY OF THE INVENTION

The present invention proposes a vapour compression refrigeration systemincluding a compressor arranged to recirculate refrigerant through acondenser, an expansion device and an evaporator, in which relativelyhot refrigerant from the condenser flows through a defrost receiverbefore passing through the expansion device, and, in a defrost phase, avalve arrangement connects the evaporator to the defrost receiver tocreate a defrost circuit which allows hot fluid to pass from the defrostreceiver to the evaporator and liquid refrigerant in the evaporator toflow to the defrost receiver,

-   -   characterised in that the refrigeration system is constructed        and operated such that, in a pre-defrost phase, the valve        arrangement closes the fluid input to the evaporator and the        compressor operates to partially evacuate the evaporator before        the evaporator is connected to the defrost receiver.

By isolating the input to the evaporator prior to commencement of thedefrost phase and allowing the compressor to remove refrigerant from theevaporator, the commencement of the defrost phase causes the hotrefrigerant to boil and results in immediate flash flooding of theevaporator with hot refrigerant vapour. The invention therefore providesa means of defrosting the evaporator which uses a minimum amount of netenergy from the system and which also enables a significant reduction inthe defrost period. In food applications therefore, the inventionminimises excursions from the ideal storage temperature of the product.

BRIEF DESCRIPTION OF THE DRAWINGS

The following description and the accompanying drawings referred totherein are included by way of non-limiting example in order toillustrate how the invention may be put into practice. In the drawings:

FIG. 1 is a diagram of a known form of vapour compression refrigerationcircuit upon which the present invention is based;

FIG. 2 is a diagram of a first such refrigeration circuit incorporatinga defrost system in accordance with the invention;

FIG. 3 is a diagram of a second such refrigeration circuit incorporatinga defrost system in accordance with the invention;

FIG. 4 is a modified form of the refrigeration circuit shown in FIG. 3;

FIG. 5 is a modified form of the refrigeration circuit shown in FIG. 2which can be used with multiple evaporators; and

FIG. 6 shows a further modification as applied to the refrigerationcircuit of FIG. 5.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1, shows a widely used direct expansion arrangement to which thepresent invention may be applied, comprising a closed refrigerantcircuit in which a compressor 1 pressurises vapour phase refrigerant.The hot superheated gas leaving the compressor passes to a condenser 2in which desuperheating and subcooling occurs. The warm high pressureliquid refrigerant then passes to a liquid receiver vessel 3 acting as arefrigerant reservoir. Liquid from the reservoir supplies an expansiondevice 4 where a rapid drop in pressure produces a two phase stream ofcold vapour and liquid which then enters the bottom of evaporator 5.Evaporation of the liquid phase is completed in the evaporator so thatthe required cooling effect is achieved. Cold sub-cooled vapour from atop exit of the evaporator 5 then returns to the inlet of the compressor1 via the suction line of the compressor and the cycle is repeated.

Various embodiments of the invention will now be described which achieverapid energy-efficient defrosting of the evaporator in such arefrigeration system. In the following description and drawings thereference numbers used in FIG. 1 are applied to corresponding itemswithin the refrigeration system.

In the first embodiment which is shown in FIG. 2 a defrost receiver 6 isinserted into the liquid stream between the main liquid reservoir 3 andthe expansion device 4, which may be an expansion valve. A shut-offvalve 7 is inserted into the flow path between the receiver 3 and thedefrost receiver 6, and an isolation valve 8 is inserted between theexit of the evaporator 5 and the inlet of the compressor 1. A drainvalve 9 is connected in parallel with the expansion valve 4, and adefrost valve 10 is connected between the top of the defrost receiver 6and the exit of the evaporator 5. During normal operation the expansionvalve 4 and valves 7 and 8 are open and valves 9 and 10 are closedresulting in a refrigerant flow circuit which is essentially the same asthat shown in FIG. 1. As previously explained however, normal operationof the circuit will result in ice formation on the outside of theevaporator due to condensation of atmospheric water vapour.

When defrosting of the evaporator is required the expansion valve 4 isfirstly closed to close off the fluid inlet of the evaporator while thecompressor 1 continues to run. The suction line to the compressorcontinues to draw refrigerant vapour from the evaporator 5, causingpartial evacuation of the evaporator. After a sufficient period of time,valves 7 and 8 are closed and valve 10 is opened allowing high pressureliquid refrigerant in the defrost receiver 6 to flash over into theevaporator 5, which is at a very low pressure. (The compressor may beturned off during this phase.) Refrigerant vapour condenses in theevaporator releasing latent heat and transferring it at high heattransfer efficiency until the pressures in the evaporator 5 and thedefrost receiver 6 equalise, at which point drain valve 9 is opened toallow liquid refrigerant in the evaporator to drain back into thereceiver 6 under the action of gravity. When the temperature of theliquid in the receiver 6 falls to a predetermined level indicating thatdefrost is complete, valves 9 and 10 are closed and valves 4, 7 and 8are opened and the normal operation of the refrigeration circuitresumes.

In a further improvement of the defrost system in accordance with theinvention the heat energy extracted from the hot liquid refrigerant andmade available for defrost may be augmented by means of a phase-changeunit 11 contained within the defrost receiver 6. A suitable phase-changemedium is encapsulated within the phase-change unit 11 so that duringnormal operation the hot liquid refrigerant flows in contact with thephase-change unit melting the phase-change material and storing enthalpyfrom the liquid refrigerant stream as latent heat. During the defroststage the stored heat energy is released into the refrigerant streamcirculating in the closed loop thereby accelerating the defrost process.The result of such extraction of heat from the hot liquid refrigerantstream is to increase the thermodynamic efficiency of the overallrefrigeration circuit through a more effective expansion process, whichlargely compensates for the extra energy needed to re-cool theevaporator after a defrost. The energy cost of the defrost process isthereby minimised.

In a second embodiment of the invention which is shown in FIG. 3 theliquid reservoir 3 is arranged to act as a defrost receiver. Theevaporator is at a higher level than the receiver, and the expansiondevice 4 is of a type which can be fully opened to remove therestriction, for example an expansion valve driven by a stepper motor.An isolation valve 12 in the compressor suction line is open when thecompressor is running and closed at other times. A defrost valve 13connects the exit of the evaporator to the top of the receiver 3 and isshut in normal operation. When defrost is initiated the expansion valve4 is fully closed for a period to allow the evaporator to empty via thesuction line. The compressor 1 is then switched off and valve 12 isshut. The expansion valve 4 is fully opened allowing hot liquid to drainback to the liquid receiver, and valve 13 opens allowing vapour from thetop of the receiver 3 to flash over into the partially evacuatedevaporator. As the evaporator is above the receiver and the line fromthe receiver 3 through the expansion valve 4 is full of liquid a flowwill be established from the evaporator through the expansion valve backto the receiver 3. Vapour will continue to flow from the receiver 3through the defrost valve 13 to the evaporator 5 where it will condense,and the condensed liquid will then flow back to the receiver 3 via theexpansion valve 4.

In a variation of this embodiment a heat exchanger 14 containing a phasechange medium may be added between the receiver 3 and the expansionvalve 4. This increases the energy storage capacity while minimising therefrigerant charge. Alternatively, as shown in FIG. 4, a heat exchanger15 of the fluid-to-fluid type can be used. The secondary of the heatexchanger is connected to a pump 16 which circulates an antifreeze fluidfrom a separate tank 17 in a closed circuit, thus acting to increase thethermal storage capacity of the defrost system.

In refrigeration installations with multiple evaporators fed from commonliquid supply and suction manifolds, such as those used in supermarketdisplay cabinets or cold storage facilities, the embodiment of theinvention shown in FIG. 5 may be used. The individual evaporators 5 andassociated defrost circuitry constructed and operated as previouslydescribed in relation to FIG. 2 are each connected to the common liquidmanifold 18 and suction manifold 19. It will be noted that in this caseeach evaporator 5 is associated with its own defrost receiver 6 so thatflash defrosting of the individual evaporators may again take place asdescribed.

In the embodiments described above the evaporator 5 should be higherthan the heat store module formed by the defrost receiver 6 and thephase-change unit 11 (if provided) so that liquid refrigerant can returnto the receiver 6 under the action of gravity. FIG. 6 shows how thisrequirement can be obviated by adding a pump 20 in series with the valve9 between the liquid outlet from the evaporator 5 and the defrostreceiver 6. The pump 20 will return cold liquid refrigerant from theevaporator 5 to the heat store 6, 11 where it can evaporate and returnto the evaporator as vapour. It should also be noted that with such anarrangement the valve 9 could be replaced with a non-return valve,removing the requirement for actuation by the refrigeration controlsystem.

Although the specific embodiments described above are applied torefrigeration systems of the direct expansion type which maintain aconstant superheat at the evaporator exit, the invention can also beapplied to flooded evaporator and liquid overfeed refrigeration systems.In such systems the evaporator is fed with liquid refrigerant and filledwith boiling refrigerant so that a mixture of liquid refrigerant andrefrigerant vapour exits from the evaporator. This requires the additionof a low pressure accumulator in the suction line so that the liquid canbe separated from the vapour which is returned to the compressor.Provided the return to the accumulator is above the fluid level in theevaporator all of the liquid in the evaporator should evaporate when theliquid feed to the evaporator is turned off during the pre-defrostphase. The valve arrangement may need to be modified, but the basicprinciple of partial evacuation of the evaporator followed by flashflooding with hot refrigerant from the liquid supply line would stillapply.

In each embodiment of the invention the heat energy extracted from thehot liquid refrigerant can be augmented by means of electrical powersupplied by a resistance heater located in or around the defrostreceiver with the purpose of accelerating the defrost process.

The timing and sequencing of the valve operation, the sizing andpositioning of the defrost receiver relative to the evaporator, and theuse of thermal capacity enhancement by means of phase change materials,secondary fluid circuit or electrical power can be optimised for maximumoverall system efficiency.

The type of valves which may be employed in the refrigeration unitsdescribed above include, inter alia, check valves, solenoid valves,expansion valves and three-way valves.

The control system employed to manage the operation of the refrigerationsystems described above will initiate and terminate the defrost processbased on information supplied by temperature and pressure sensors fittedat strategic points around the refrigerant circuits.

Whilst the above description places emphasis on the areas which arebelieved to be new and addresses specific problems which have beenidentified, it is intended that the features disclosed herein may beused in any combination which is capable of providing a new and usefuladvance in the art.

1. A vapour compression refrigeration system including a compressor (1)arranged to re-circulate refrigerant through a condenser (2), anexpansion device (4) and an evaporator (5), in which relatively hotrefrigerant from the condenser flows through a defrost receiver (6/3)before passing through the expansion device, and, in a defrost phase, avalve arrangement (10/13, 9, 4) connects the evaporator to the defrostreceiver to create a defrost circuit which allows hot fluid to pass fromthe defrost receiver (6/3) to the evaporator (5) and liquid refrigerantin the evaporator to flow to the defrost receiver, characterised in thatthe refrigeration system is constructed and operated such that, in apre-defrost phase, the valve arrangement (9, 4) closes the fluid inputto the evaporator (5) and the compressor (1) operates to partiallyevacuate the evaporator (5) before the evaporator is connected to thedefrost receiver (6/3).
 2. A vapour compression refrigeration systemaccording to claim 1 in which the hot refrigerant from the condenser (2)is stored in the defrost receiver (6/3) before passing through theexpansion device (4) and, at the commencement of the defrost phase, thestored refrigerant is admitted to the evaporator through a defrost valve(10/13).
 3. A vapour compression refrigeration system according to claim1 in which energy from the hot liquid refrigerant obtained from thecondenser (2) is stored in a phase-change medium (11/14) which is inheat-exchange contact with the hot liquid, and the stored energy isreleased into the defrost fluid flowing through the evaporator (5)during the defrost phase.
 4. A vapour compression refrigeration systemaccording to claim 3 in which the phase-change medium (11) is containedwithin the defrost receiver (6).
 5. A vapour compression refrigerationsystem according to claim 3 in which the phase-change medium (14) isincluded between the defrost receiver (3) and the expansion device (4).6. A vapour compression refrigeration system according to claim 1 inwhich a fluid-to-fluid heat exchanger (15) is included between thedefrost receiver (3) and the expansion device (4) and a heat storagefluid is circulated through the secondary of the heat exchanger to astorage reservoir (17).
 7. A vapour compression refrigeration systemaccording to claim 1 in which heating means is arranged to provideadditional heat input to the hot refrigerant flowing from the condenser(2).
 8. A vapour compression refrigeration system according to claim 1which includes a plurality of evaporators (5) and in which eachevaporator is associated with a respective defrost receiver (6).
 9. Avapour compression refrigeration system according to claim 1 in which apump (20) is arranged to return liquid refrigerant from the evaporator(5) to the defrost receiver (6) during the defrost phase.
 10. A methodof defrosting a vapour compression refrigeration system including acompressor (1) arranged to re-circulate refrigerant through a condenser(2), an expansion device (4) and an evaporator (5), in which relativelyhot refrigerant from the condenser flows through a defrost receiver(6/3) before passing through the expansion device, and, in a defrostphase, a valve arrangement (10/13, 9, 4) connects the evaporator to thedefrost receiver to allow hot fluid to pass from the defrost receiver(6/3) to the evaporator (5) and liquid refrigerant in the evaporator toflow to the defrost receiver, characterised in that the refrigerationsystem is constructed and operated such that, in a pre-defrost phase,the valve arrangement (9, 4) closes the fluid input to the evaporator(5) and the compressor (1) operates to partially evacuate the evaporator(5) before the evaporator is connected to the defrost receiver (6/3).